A wide range of technical solutions was proposed in the past for achieving high three-dimensional spatial resolution in imaging radiation detectors, preferably gamma detectors, while assuring high detection efficiency. Achieving high resolution in Single Photon Computerized Tomography (SPECT) when using pinhole collimators or in coincidence Positron Emission Tomography (PET) detectors, requires not only high standard resolution in the plane of the detector module but also accurate information on the depth-of-interaction (DOI) of the converting photons inside the active material such as crystal scintillator. Common aspect of these both situations (pinhole SPECT and PET) is that there are many inclined radiation trajectories involved and to achieve high resolution operation, the information on the third—depth—coordinate of the point inside the scintillator module where the radiation, preferably gamma ray, interacted/converted, is necessary. Many technical approaches were developed in the past to achieve high DOI resolution and are for example described in the referenced review papers [1-5]. In the current invention, we propose a hybrid scintillator configuration for the scintillation crystals forming the scintillation detection block.
Most scintillating crystal blocks for high spatial resolution detectors, preferably gamma ray detectors, such as the ones used in small animal PET scanners, use a configuration based on crystal pixel arrays (for example Naviscan's breast PET imager, Siemens microPET and INVEON small animal scanners, LabPET small animal PET from TriFoil imaging, etc). A new recent design based on continuous (also known as monolithic) crystal slabs has also shown excellent position resolution (Oncovision's MAMMI PET breast imager and ALBIRA small animal PET imager, University of Washington research effort, and Delft University in Holland research project). However, both pixelated and monolithic configurations present important limitations for thick (around 15 mm or more in the PET detector) crystal blocks, required for stopping (and hence detecting) with high efficiency the high energy annihilation 511 keV photons emitted by the PET radioisotopes. Crystal pixel arrays do not offer DOI information and when the pixels are very thin and long they provide poor energy and timing resolution and are costly to manufacture. Monolithic crystal slabs show poor spatial resolution at the edges and near the entrance surface when using thick crystals.
Therefore, there are important drawbacks in the current technology: to solve these problems, in particular the conflict between the thickness-dependent detection efficiency of the monolithic crystal and the poor energy and timing resolution of the crystal pixel arrays, in this invention, we propose a hybrid configuration in which the scintillation block is a hybrid scintillation module formed by a component (or multiple components) of crystal pixel arrays coupled to a component (or multiple components) of a continuous crystal slab (or several slabs). Such hybrid configuration overcomes the mentioned limitations by providing high stopping power (the stopping power can be defined as the probability for the incoming radiation to deposit most of its energy in the target material (dE/dx) with high 3D resolution of the conversion position of the radiation, preferably gamma ray, inside the whole crystal block, and with good energy and timing resolutions, at the same time.
It is important to clarify that the term “hybrid detector” was also used in the past in other contexts. In an example of one such use of the term “hybrid” (U.S. Pat. No. 6,819,738: “Hybrid scintillator/photo sensor & direct conversion detector”) it describes the combination of two or more different radiation detection modality systems into one combined CT system. Other examples are PET/CT or PET/MRI imager combinations where two modalities operate as one system. Another use of the “hybrid” term is given to the radiation sensor that functions as two types of modalities, for example to detect gamma and X-ray radiation and produce emission (gamma) and transmission (X-ray) images with the same radiation sensor.
U.S. Pat. No. 6,946,841-B2 discloses an apparatus for combined nuclear imaging and magnetic resonance imaging, and method thereof. A combined MR and nuclear imaging device comprising an MRI device and a nuclear imaging device, wherein the nuclear imaging device is capable of operating with the magnetic field of the MRI device or in a region where the magnitude of the magnetic field is lower. The combined system allows MRI examination and nuclear medicine examinations to be conducted quasi-simultaneously with no, or minimal, motion of the patient during the combined examination. The nuclear imaging device comprises nuclear detector modules capable of operating within a large magnetic field in the bore of the MRI scanner when the modules are oriented in the direction of the field, and are capable of operating in any direction when the magnitude of the field is below a certain threshold.
Other efforts listed below refer to the crystal treatment and coupling methods. U.S. Pat. No. 6,713,767-B2: “Hybrid two-dimensional scintillator arrangement” describes a scintillator arrangement comprising: a plurality of detector strips comprising a plurality of scintillator slabs, the scintillator slabs being separated from one another by absorber layers; and a fitting form wherein at least two of said plurality of detector strips are arranged essentially parallel to one another in said fitting form and wherein the fitting form comprises transverse pieces and a frame, the transverse pieces being arranged essentially parallel to one another and being attached to opposite sides of the frame. In our invention there are not absorbers in between the scintillators: the light is transferred from one slab to another. That design is only addressing the issue of two dimensional and not three dimensional arrangement. The absorber layers disclosed in U.S. Pat. No. 6,713,767 introduce asymmetry in the transversal light propagation and would decrease the performance in case they would be used in our structure. Except for the name, these two structures are entirely different.
U.S. Pat. No. 3,978,336-A discloses a hybrid scintillation scanning apparatus” that comprises: a scintillation crystal bar of elongated form receiving the gamma radiation through a collimator and comprising a transparent upper face optically coupled by means of a light guide to a plurality of photomultipliers. The photomultipliers furnish their signals to an electronic combining circuit supplying, on one hand, an amplitude analyzer and, on the other hand, a computing circuit permitting the location of the scintillation along the axis of the bar. The scanning apparatus disclosed in this patent does not include a combination of scintillation modules: one continue scintillation module and one pixelated scintillation module as the present invention does. It is therefore a completely different object.
U.S. Pat. No. 7,692,156-B1 “Beam-oriented pixelated scintillators for radiation imaging” discloses a radiation detection device, comprising: a two-dimensional, beam-oriented pixelated scintillator, the scintillator having a first pixel having a first pixel axis and a second pixel having a second pixel axis, wherein the first and second axes are at an angle relative to each other, and wherein each axis is substantially parallel to a predetermined beam direction for illuminating the corresponding pixel. According to particular embodiments, the device of U.S. Pat. No. 7,692,156-B1 the scintillator comprises inter-pixel grooves and the pixels have a two dimensional monolithic array. This is entirely different from the two component scintillation module of the present invention. We are not using inter-pixel groves.
CN102707310 discloses a device wherein “the scintillation crystal array are built by strip-type scintillation crystals arranged along width and length directions” However, in addition to other advantages, there is an essential difference between the hybrid scintillation module of claim 1 of the present application, wherein the multiplicity of scintillators of the scintillation array are pixelated, whereas according to D1 the scintillation crystals are “strip-type”.
Pixels and strips are completely different technical terms: pixels are two-dimensional, whereas strips are one-dimensional (of course they have width and length as parameters but due to their shape the operational extraction of spatial information is different). Pixels can be interconnected in readout strips, but when starting with strips some spatial information is irreversibly already lost.
Meaning of Some Terms and Expressions Used in this Application
The expressions “monolithic crystals”, “monolithic scintillation slab”, “monolithic scintillation plate”, “continuous crystal slab”, “continuous scintillation plate”, “continuous scintillator plate” and monolithic scintillator are used indistinctly along this specification.
The expressions “pixelated scintillator array”, “pixelated scintillation array”, and “pixelated scintillation plate” are used interchangeably.
In this application the expression “scintillation module”, “scintillator module”, and “scintillation block” are used interchangeably.